A field effect transistor (FET) is used in a switching power supply device which is used as an alternating current (AC)/direct current (DC) converter or a DC/DC converter, and in a semiconductor device such as an amplifier.
For instance, a silicon (Si)-metal-oxide-semiconductor (MOS) FET is widely used in an FET. In recent years, a high electron mobility transistor (HEMT) has been developed. As a typical HEMT, a GaN-HEMT is known which uses a gallium nitride (GaN)-based compound semiconductor having a low on-resistance.
Meanwhile, the current of an FET varies with the ambient temperature. When the current of an FET varies, a malfunction of a circuit which uses the current may be caused. The threshold voltage of an FET also varies with the ambient temperature. In a method of reducing temperature rise of an FET, the temperature of the FET is detected by a thermocouple, and when the temperature of the FET increases, the volume of air applied to heat radiating fins installed in a device including the FET is increased.
In a switching power supply device, control is performed to minimize the variation in the output voltage and the current of an FET by changing the duty ratio of a gate voltage (pulse voltage) of the FET using a control integrated circuit (IC) according to the output voltage and the current of the FET.
The change of the duty ratio of the gate voltage as described above is effective when the variation in the output voltage of a switching power supply device is reduced. However, for a variation in the current of the FET, the gate voltage outputted from the control IC has uniform maximum voltage and minimum voltage, thus even when the duty ratio is changed, the effect of reducing the variation in the current is small.
The followings are reference documents.
[Document 1] Japanese Laid-open Patent Publication No. 2006-87215 and
[Document 2] Japanese Laid-open Patent Publication No. 05-19879.